In general, an optical biometric capture device comprises an optical sensor such as a camera and an electronic control unit connected to the sensor in order to control it. The control unit is arranged to control image capture by the sensor, image processing, and extracting characteristics concerning the captured body part for the purposes of:                comparing them with characteristics stored in the device or imported via secure external communication; or        exporting them to a comparator device via secure communication.        
The comparison serves to identify the person to whom said body part belongs or to verify that the characteristics of the body part appearing in the image do or do not belong to one of the people authorized to access a premises or a service. A dishonest person may attempt to act on the control unit to access its comparison algorithms or its encoding algorithms used for enciphering the characteristics prior to exporting them, or more generally to access operating parameters of the control unit. The control unit is therefore one of its central components of the device, and it needs to be made secure in order to prevent any action being taken thereon by a non-authorized person seeking to falsify identification or authentication.
It is known for the control unit to have incorporated therein a passive protection element such as a resin, or an active protection element such as a resistive film or mesh (see for example documents FR-A-2 805 074 and FR-A-2 824 697) so that any unauthorized physical alteration or manipulation on the control unit leads to an alteration of the protection element that reveals the intervention. Nevertheless, that solution is not practical, for example because it introduces constraints on the design of the device (overall size, position), of compatibility with light rays, and of passages for cables or electrical harnesses, that require discontinuities in the protection element and thus potentially provide access to the protected elements.